Ask the authors if they could put an acknowledgement in the paper or ask for a letter of acknowledgement (something many cores do, even if you're not officially a core) so later on in your career you can show someone or put it on your CV. Be polite, I can tell you right now they're not going to come back if the dialogue gets sour and you'll probably piss off your boss if you start a fight over this. Don't forget that a free figure today could be the start of a few authorships tomorrow. As others have said, don't burn a bridge over a toothpick.
Be polite and don't be aggressive, they don't have to put you on the paper, end of discussion. This is a tough lesson, doing work for something that gets published isn't always a ticket to getting authorship, and it isn't supposed to.
Here's what the NIH has to say:
"For each individual the privilege of authorship should be based on a significant contribution to the conceptualization, design, execution, and/or interpretation of the research study, as well as a willingness to take responsibility for the defense of the study should the need arise. In contrast, other individuals who participate in part of a study may more appropriately be acknowledged as having contributed certain advice, reagents, analyses, patient material, support, etc., but not be listed as authors. It is expected that such distinctions will be increasingly important in the future and should be explicitly considered more frequently now."
This brings up a few things that people tend to forget. Authorship isn't a list of all of the people involved in the work and most certainly isn't intended for everyone who carried out any technical work. The ideas that an author is responsible for the work and will take up its defense is often forgotten. I don't think that this is perfect, but I tend to agree with most of it.
However, I do think technical staff should get authorships when they generated the bulk of the data or simply did an exhausting amount of work. I also think that the person should have the intent and ability to "give a shit". Plenty of people do science as a job (nothing wrong with that) and don't really have an interest in the low-yield life of academic science. Also, if the person has the best intentions but will likely be unable to be present during review/defense/etc because of their primary obligations and responsibilities, then they can't be on the paper.
I don't want to be rude here, but I doubt the coverage plot was required for, or was in its self, an advancement in science. I'd bet a dollar it was probably something they asked you to cook up to address the whining of a reviewer. It sounds like you are finding out that they used the figure after the article was published, which means they asked for it during review. (Please correct me if I'm wrong)